The present invention relates generally to magnetic resonance (MR) imaging and, more particularly, to a method and apparatus for multi-dimensional parallel MR imaging. The invention is further related to accelerated data acquisitions in at least two simultaneous directions. In this regard, the invention supports acceleration in the cardinal directions as well as multi-oblique imaging planes.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal may be received and processed to form an image from the excited spins after the excitation signal B1 is terminated.
Accelerated MR imaging techniques have been developed to expedite MR data acquisition thereby reducing scan time and increasing subject throughput. One known accelerated MR imaging technique is parallel imaging whereby a phased coil array samples an imaging volume. Generally, scan time reduction is achieved by under-sampling k-space and recording images simultaneously from multiple imaging or receive coils. Under-sampling generally reduces the data acquisition time and the use of multiple receive coils, such as a phased coil array, reduces wraparound caused by under-sampling. In this manner, scan time is reduced by increasing the distance of sampling positions in k-space. If an image space is under-sampled in the phase encoding direction, for example, by a factor of two, then it will take half the time to acquire the image. In this regard, every pixel in the image will represent data from two spatial points.
With known parallel imaging techniques, such as SENSitivity Encoding (SENSE), coil sensitivity data is reacquired. The coil sensitivity data is generally acquired in a low spatial resolution scan and is used to ascertain the sensitivity of each coil of the receive coil array to a field-of view (FOV). Generally, the coil sensitivity data is used to weight the imaging data such that coil sensitivity is reflected in the reconstructed image, and, as a result, the coil sensitivity data reduces aliasing in the reconstructed image that can occur as a result of under-sampling. Like other parallel imaging techniques, SENSE utilizes under-sampling for accelerated data acquisition. Despite the advantages that have been achieved using known parallel imaging techniques and receive coils, scan time reduction has been limited because known receive coils are only capable of acceleration in one direction. This has been shown to be particularly problematic in brain imaging.
Parallel imaging is increasingly being used for accelerated imaging of the brain and other anatomical areas of interest. The challenges facing parallel imaging include available baseline signal-to-noise ratio (SNR), noise amplification, and acceleration factor. As described above, current coil construction supports acceleration in only one dimension. Moreover, coil designs to reduce noise have been unable to improve acceleration factors without affecting SNR. For instance, a circular array comprising sixteen coil elements distributed circumferentially has been suggested to alleviate noise amplification. However, it has been shown that electro-dynamic constraints dictate a fairly rapid degeneration in SNR at high 1D accelerations. Additionally, by placing a multitude of coil elements in the tangential direction, the individual elements distributed circumferentially get extremely narrow and long in size to cover the entire brain. Such narrow elements have been shown to become coil noise dominated, as a function of the proximity of the subject, the frequency of the coil, and the temperature of the coil.
It would therefore be desirable to have a method and apparatus for acceleration in more than one direction to further reduce scan time without sacrificing SNR.